Electronic devices commonly require a direct current (DC) voltage of appropriate level for proper operation. A manufacturer of an electronic device provides for a power signal to be connected to the electronic device, which is used to power the semiconductor packages and other electric components of the electronic device. In many cases, the provided power signal is at a different voltage potential than the voltage required to operate the individual components of the electronic device. The manufacturer will generally provide a power conversion circuit to generate a steady DC voltage signal at a voltage potential usable by the electronic device. Switch-mode power supplies (SMPS) are common due to efficiency advantages.
An SMPS may be located within an electronic device, or located externally and connected to the electronic device by a cable. The cable is coupled to the electronic device using a detachable plug in some embodiments. The plug may include both power and data lines, e.g., when an electronic device is a cell phone, tablet computer, or other mobile device, and power is provided by a Universal Serial Bus (USB) interface. In some embodiments, electronic devices follow the USB Power Delivery (USB-PD) protocol to negotiate a voltage potential for power delivery by an external SMPS.
An SMPS operates by switching an input power signal on and off repeatedly to create a relatively high-frequency power signal. The switched power signal is routed through a transformer or inductor, and then rectified and filtered to create a steady DC power signal. The output power signal is commonly rectified by one or more diodes, or a transistor is used for synchronous rectification.
A circuit diagram for one exemplary embodiment of a flyback SMPS 100 is illustrated in FIG. 1a. SMPS 100 is formed by components disposed on a circuit board, PCB, or other substrate 101. In some embodiments, SMPS 100 is split across multiple substrates 101. SMPS 100 is split into a primary side 102 and a secondary side 104, which are delineated by transformer 105. In non-isolated topologies, an inductor is used instead of transformer 105. Transformer 105 includes a primary winding 106 as part of primary side 102 and a secondary winding 108 as part of secondary side 104. Primary side 102 of SMPS 100 is made up of the components electrically connected to primary winding 106. Secondary side 104 of SMPS 100 is made up of the components electrically connected to secondary winding 108. Transformer 105 provides DC isolation between primary side 102 and secondary side 104. Alternating current (AC) signals through primary winding 106 are transferred to secondary winding 108 by magnetic coupling, while any DC offset is substantially ignored.
Primary side 102 includes a power input at bulk voltage (VBULK) node 110. In some embodiments, VBULK node 110 receives an AC power signal provided by an electric utility at, e.g., 110 or 230 volts AC, which has been rectified. The AC electric signal is routed to a residence, commercial office building, or other premises by power mains, and input to the electronic device including SMPS 100 by plugging the device into a wall outlet. A diode bridge or other rectifier circuit rectifies the input AC mains signal to include positive voltage values at VBULK node 110. In other embodiments, a power signal is provided to VBULK node 110 by other means, e.g., from solar cells or a battery pack. A capacitor 111 is coupled between VBULK node 110 and ground node 113 to further filter the input power signal. Ground node 113 operates as the ground reference voltage for the electrical components of primary side 102.
Electric current from VBULK node 110 through primary winding 106 to ground node 113 is turned on and off by primary MOSFET 112. Primary MOSFET 112 includes a drain terminal coupled to primary winding 106 opposite VBULK node 110, a gate terminal coupled to primary flyback controller 120 at circuit node 114 (DRV), and a source terminal coupled to current sense resistor 118 at current sense (CS) node 119. The source and drain terminals of primary MOSFET 112 are conduction terminals, and the gate terminal is a control terminal. Controller 120 turns on, or enables electric conduction through, primary MOSFET 112 by providing a positive voltage potential at the gate terminal of the MOSFET via DRV node 114 coupled to a drive output of the controller. In some embodiments, additional driver circuitry is coupled between controller 120 and the gate of MOSFET 112. When primary MOSFET 112 is turned on, electric current flows from VBULK node 110 to ground node 113 through primary winding 106, primary MOSFET 112, and resistor 118 in series. Controller 120 turns off primary MOSFET 112 by outputting a ground voltage potential to the gate of primary MOSFET 112. While primary MOSFET 112 is off, no significant current flows from VBULK node 110 through primary winding 106.
In the ideal case, an n-channel MOSFET exhibits zero resistance when its gate has a positive voltage potential, and exhibits infinite resistance when its gate is at ground potential. MOSFET 112 is an n-channel MOSFET that operates as a switch opened and closed by a control signal from controller 120 coupled to the MOSFET's gate terminal at DRV node 114. A switch, e.g., MOSFET 112, being closed is also referred to as the switch being “on,” because electric current is able to flow between conduction terminals of the switch. An open switch is referred to as being “off” because current does not flow significantly between the conduction terminals of the switch. While the primary switch of SMPS 100 is illustrated as an re-channel MOSFET, other types of electronically controlled switches, e.g., bipolar-junction transistors (BJTs), p-channel MOSFETs, gallium arsenide transistors, junction gate field-effect transistor, other types of field-effect transistors (FETs), and other types of electronic switches, are used in other embodiments. FETs include source and drain terminals, which are conduction terminals, and a gate terminal as a control terminal. BJTs include emitter and collector terminals, which are conduction terminals, and a base terminal as a control terminal.
Controller 120 determines when to switch primary MOSFET 112 by observing the magnitude of current through primary winding 106. Resistor 118 creates a voltage potential difference between ground node 113 and CS node 119 when electric current flows through the resistor. The voltage potential across resistor 118, as observed at CS node 119, is approximately proportional to the current through primary winding 106. CS node 119 is coupled to a current sense input pin of controller 120. Controller 120 observes the voltage potential at CS node 119 to determine the electric current magnitude through primary winding 106.
While controller 120 has primary MOSFET 112 turned on, electric current through primary winding 106 increases approximately linearly and magnetizes transformer 105. When controller 120 turns off primary MOSFET 112, electric current through primary winding 106 is substantially stopped. The magnetic energy stored in transformer 105 while MOSFET 112 is closed is output as electric current through secondary winding 108 while MOSFET 112 is open, creating a positive voltage potential at voltage output (VOUT) node 124 relative to ground node 126. Ground node 126 operates as the ground reference voltage for electrical components of secondary side 104. SMPS 100 is an isolated topology, meaning a separate primary side ground node 113 and secondary side ground node 126 are used. The voltage potential of ground node 126 is allowed to float relative to ground node 113.
The voltage potential at VOUT node 124 charges capacitor 128 and powers additional circuit components of an electronic device connected to SMPS 100 as a load. The cycle repeats when controller 120 turns on primary MOSFET 112 to again magnetize transformer 105. Capacitor 128 provides power to VOUT node 124 while primary MOSFET 112 is on, and transformer 105 is being magnetized. Diode 130 rectifies current through secondary winding 108 by reducing electric current flowing from VOUT node 124 to ground node 126 through secondary winding 108 while transformer 105 is being magnetized from primary side 102.
Feedback is provided from secondary side 104 to primary side 102 by Zener diode 154 and optocoupler 155. Optocoupler 155 includes an LED 156 and a phototransistor 158. If the voltage potential at VOUT node 124 exceeds the Zener voltage of Zener diode 154 summed with the voltage drop of LED 156, current flows from VOUT node 124 to ground node 126 through Zener diode 154 and LED 156 in series. Photons emitted by LED 156 hit phototransistor 158, which turns on the phototransistor and increases the coupling of feedback (FB) node 160 to ground node 113. FB node 160 is coupled to a feedback input pin or terminal of controller 120. Capacitor 159 filters the voltage potential at FB node 160. As current through LED 156 is increased, the coupling of FB node 160 to ground node 113 through phototransistor 158 is increased, and the voltage potential of FB node 160 is further reduced.
As controller 120 observes voltage potential at FB node 160 being reduced, the controller understands that voltage potential at VOUT node 124 is at or above a desired output voltage potential. Controller 120 takes measures as configured to reduce power transfer from primary side 102 to secondary side 104, e.g., reducing on-time of MOSFET 112 or modifying the switching frequency of DRV signal 114.
Optocoupler 155 in FIG. 1a provides feedback from secondary side 104. In other embodiments, as shown with SMPS 166 in FIG. 1b, feedback is provided from primary side 102 using an auxiliary winding 168 of transformer 105. Auxiliary winding 168 is wrapped around a common magnetic core 169 with primary winding 106 and secondary winding 108. Auxiliary winding 168 is rectified by diode 170 to charge capacitor 171 and generate an auxiliary voltage at VDD node 202. The auxiliary voltage at VDD node 202 is routed to provide power to controller 120. When the auxiliary voltage at VDD node 202 exceeds the Zener voltage of Zener diode 172, a feedback current is generated through resistors 176 and 178 to ground node 113.
Auxiliary winding 168 is referenced to ground node 113, so separate isolation by optocoupler 155 is not required. However, the voltage provided by resistors 176 and 178 has a direct relationship with the current output by SMPS 166 at VOUT node 124, rather than the inverse relationship of FB node 160 with optocoupler 155 in FIG. 1a. Coupling the anode of Zener diode 172 to FB node 160 would result in current being injected into controller 120, whereas phototransistor 158 results in electric current drawn from controller 120.
To use the same controller 120 integrated circuit with the primary side feedback in FIG. 1b as with the secondary side feedback in FIG. 1a, and with non-isolated SMPS topologies, the feedback current from auxiliary winding 168 is inverted by BJT 174. Resistors 176 and 178 create a voltage divider to calibrate the voltage potential at the base terminal of BJT 174 for a given current through Zener diode 172. Capacitor 182 filters the voltage potential at FB node 160.
BJT 174 creates an inverse relationship between VOUT node 124 and the collector terminal of BJT 174 coupled to FB node 160, so that the same controller 120 feedback pin can be used with secondary side feedback, as in FIG. 1a, and primary side feedback, as in FIG. 1b. With either secondary side feedback or primary side feedback, the feedback signal received by controller 120 at FB node 160 has an inverse relationship with VOUT node 124. However, a BJT 174 external to controller 120 is used to invert the feedback signal with primary side feedback. The additional hardware components required when primary side feedback is used, or in non-isolated topologies, increases the cost of SMPS 166 and increases the overall design footprint.